1. Field of the Invention
The present invention relates to a perpendicular magnetic recording type magnetic recording/reproducing apparatus and a perpendicular magnetic recording medium used in the apparatus.
2. Description of the Related Art
Conventionally, in a perpendicular double-layered medium in which a perpendicular magnetic recording layer is multilayered on a high-permeability soft magnetic layer, this soft magnetic layer takes part of the function of a recording head, i.e., passes a recording magnetic field, which is generated from a magnetic head and which magnetizes the perpendicular magnetic recording layer, in the horizontal direction and returns this magnetic field to the magnetic head, thereby improving the recording/reproduction efficiency. On the other hand, the soft magnetic layer readily forms magnetic domains because it reduces the magnetostatic energy. A magnetic wall is formed in the boundary between these magnetic domains, and spike noise is generated in a region in which this magnetic wall exists. To suppress this spike noise, it is only necessary to suppress the formation of the magnetic wall in the soft magnetic layer. As a method of suppressing the formation of this magnetic wall, as described in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 5-258274, a large number of media have been proposed in which a hard magnetic layer is formed below a soft magnetic layer to fix the direction of easy magnetization in this soft magnetic layer in the radial direction.
In these media, however, the soft magnetic layer is directly formed on the hard magnetic layer, so the exchange coupling interaction acts more than necessary between these layers. The soft magnetic layer largely changes the magnetization direction even when applied with a weak external magnetic field of, e.g., about 7,900 A/m.
In this case, the stronger the coupling with the soft magnetic layer, the more easily the hard magnetic layer is influenced by the soft magnetic layer. As a consequence, the magnetization direction, i.e., the direction of the easy axis in the hard magnetic layer changes from the radius direction. That is, a so-called pinning failure phenomenon occurs. In effect, in a magnetic recording/reproducing device (HDD) a voice coil motor (VCM) which uses a permanent magnet to drive an arm is positioned close to a magnetic disk. A leakage flux generated from this VCM has a magnitude of about 7,900 A/m even in the vicinity of the edge of the magnetic disk. Therefore, if the above-mentioned medium is used in the HDD, pinning failure sometimes occurs as a particularly serious problem.
The use of the hard magnetic layer is convenient to control the direction of the easy axis in the soft magnetic layer. However, the hard magnetic layer is basically the same as a longitudinal magnetic recording medium, i.e., it generates medium noise. The thickness of the soft magnetic layer is preferably small in order to prevent the generation of dust during film formation and reduce the cost. However, the smaller the thickness of the soft magnetic layer and the shorter the distance between the hard magnetic layer and the magnetic head, the larger the noise from the hard magnetic layer. One means for reducing the noise from this hard magnetic layer is to reduce its thickness. Reducing the thickness of the hard magnetic layer is also advantageous in improving the film formation process and in the cost. If, however, the exchange coupling acts strongly, pinning failure easily occurs.